Deflection amplifier



4 Nov. 18, 1969 G. YANJSHEVSKY ET AL 3,479,553

DEFLECTION AMPLIFIER Original Filed Sept. 22, 196'? I g, m

DTFFERENTIAL DIFFETTENTIAL AMPLIFIER AMPLIFIER INVENTORS. GILBERTYANISHEVSKY BY BRUCE J. MILLER i/ STANLEY J. gVlNESE AGENT United StatesPatent 3,479,553 DEFLECTION AMPLIFIER Gilbert Yanishevsky, Philadelphia,Bruce J. Miller, Malvern, and Stanley J. Savinese, Ridley Park, Pa.,assignors to Burroughs Corporation, Detroit, Mich., a corporation ofMichigan Continuation of application Ser. No. 669,848, Sept. 22, 1967.This application Nov. 5, 1968, Ser. No. 774,593 Int. Cl. H013 29/10 U.S.Cl. 315-18 Claims ABSTRACT OF THE DISCLOSURE A deflection amplifierincluding cascaded differential amplifiers is connected through a bufferto the input of an inverting amplifier. The output of the invertingamplifier is connected to separate plus and minus drivers which in turndrive a deflection yoke through a push-pull output stage. Seriesconnected, forward biased diodes are included in the inverting amplifiercircuit for maintaining a constant compensating voltage differencebetween the inputs to the drivers. The gain and frequency response ofthe amplifier are controlled by a degenerative feedback path.

BACKGROUND OF THE INVENTION This case is a continuation of applicationSer. No. 669,- 848, filed Sept. 22, 1967, now abandoned.

This invention relates generally to amplifiers and more particularly tosolid state amplifiers which are especially useful in driving thedeflection yoke of a magnetically controlled cathode ray tube.

The deflection of a cathode ray tube beam is generally performed by theuse of either one of two common types of circuits, i.e., electrostaticor magnetic circuits. Electrostatic deflection requires the use of highvoltage circuitry and power supplies which technically preclude the useof solid state components such as transistors therein. The vacuum tubecircuitry necessary in the system is bulky, heat generating and powerconsuming. Also, extreme caution is required in service, repair andmaintenance because of the high voltages present and fragile components.

Magnetic beam deflection circuits have been known for some time butinvolve certain peculiar deficiencies. The circuits of this type usingvacuum tubes are not suitable for random high speed beam deflectionbecause of their long settling times. Solid state circuits such as thatdisclosed in Patent No. 3,303,380 to J. L. Kozikowski and assigned tothe same assignee as the present invention have been used for magneticbeam deflection systems. Until now however, they have been capable ofoperation over only a relatively narrow bandwidth and have required highoutput currents since they are not suitable for driving high inductanceyokes over a wide bandwidth. Furthermore, many of them are extremelyexpensive. Anoher serious difficulty which has been experienced indeflection amplifiers of this type is a lack of linearity as the outputpasses through Zero.

OBJECTIVES AND SUMMARY OF THE INVENTION It is therefore an object ofthis invention to improve solid state amplifiers suitable for the randomhigh speed magnetic deflection of a cathode ray tube beam.

A further object of this invention is to provide a lowcost solid statemagnetic beam deflection circuit operable over a wide bandwidth.

A further object of this invention is to provide a solid state magneticbeam deflection circuit which is able to drive relatively highinductance yokes.

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A still further object of the invention is to provide a magneticdeflection amplifier which is linear over its range.

In accordance with these and other objects of the invention there isprovided a solid state amplifier suitable for use in magnetic highspeed, random cathode ray tube beam deflection systems comprising highinput impedance means for providing voltage and current amplification ofinput signals and push-pull connected output means including twoopposite conductivity transistors having their emitters connected to oneside of an output impedance for supplying output current to saidimpedance. Separate transistor driver means are connected to said outputtransistors for supplying input drive current to said outputtransistors. The inputs of the driver circuits are coupled to the outputof the voltage and current amplification means. Means are included inthe current and voltage amplification means for maintaining asubstantially constant voltage difference between the inputs of saidseparate driver means for compensating for voltage drops in said driverand output means transistors. Means are also provided for degenerativelyfeeding back to the input of the current and voltage amplification meanssignals having substantially the same waveshape as the deflectingcurrent.

Various other objects and advantages and features of the invention willbecome more fully apparent in the following specification with itsappended claims and accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING The single figure of the drawing showsa circuit embodying the invention, shown partially in block diagram formand partially in schematic form.

DETAILED DESCRIPTION The invention can best be understood by referringto the following detailed description of the illustrated embodiment.

In electronic character generating and display apparatus it is desirableto be able to position the character or symbol at random, i.e., at anydesired spot on the display device such as a cathode ray tube screen.The character or symbol may be located at the bottom, middle or, top,left or right, or at any other intermediate position or location chosenfor the particular display. The system which produces the symbol orcharacter must not only coarse locate the character on the screen of thetube, but must also have suflicient frequency response to clearly traceout (form) the character at the spot chosen for its display. Twodeflection systems are required, one for horizontal and one for verticalbeam deflection.

In a system in which the apparatus of this invention may be employed,the signal input is a voltage, while the output signal is a current. Theoutput load in this instance may be a yoke coil of a cathode ray tube.It is desirable to produce as linear a relationship as possible betweenthe input voltage and the output current in the yoke since this type ofapparatus involves relatively high currents and low impedances. Thesolid state design approach lends itself admirably to this type ofcircuit design.

Referring now to drawing the symbol and coarse position signal inputsare connected to one input of differential amplifier 11 throughresistors 15 and 13, respectively. The other input of differentialamplifier 11 is connected to ground through resistor 14. Thedouble-ended outputs of differential amplifier 11 are connected to theinputs of differential amplifier 19 whose output is coupled to the baseof emitter follower connected NPN transistor 21 of buffer circuit 23.The collector of transistor 21 is grounded and the emitter is connectedto the ungrounded side of regulator capacitor 25 through resistor 27.The output of buffer circuit 23 is taken from the emitter of PatentedNov. 18, 1969 3 transistor 21 which is coupled to the base of NPNtransistor 29 of amplifier circuit 31.

One side of voltage regulator capacitor 33 is grounded and the otherside is connected to voltage source +V through voltage dropping resistor35. One end of voltage regulator capacitor 25 is grounded while theother end is connected to voltage source V through voltage droppingresistor 37.

The emitter of transistor 29 is connected to the ungrounded side ofregulator capacitor 25 through resistor 39. The collector of transistor29 is connected to the ungrounded side of regulator capacitor 33 throughdiodes 41, 43, 45 and resistor 47 all of which are connected in series.The amplifier circuit 31 has two outputs taken from either side of theset of series connected diodes 41, 43 and 45. The negative-going outputsignals are taken from the collector connected end of the series diodeswhich is coupled to the base of PNP transistor 49 of minus drivercircuit 51. Roll off capacitor 53 is connected between the base oftransistor 49 and ground. The collector of transistor 49 is connected tothe voltage source V through resistor 55 while its emitter is connectedto the ungrounded side of regulator capacitor 33 through resistor 57.

The positive-going output signals from amplifier 31 are taken from theother output of amplifier 31 which is coupled to emitter-followerconnected NPN transistor 59 of plus driver circuit 61. The collector oftransistor 59 is connected to voltage source +V through resistor 63while its emitter is connected to the ungrounded side of regulatorcapacitor 25 through resistor 65. The outputs of the plus and minusdriver circuits are taken from the emitters of transistors 59 and 49,respectively, which are connected to the bases of push-pull connectedtransistors 67 and 69, respectively, of output circuit 71. The collectorof transistor 67 is connected to voltage source -+V through seriesconnected resistor 73 and fuse 75. The collector of transistor 69 isconnected to voltage source V through series connected resistor 77 andfuse 79. The output of the output stage is taken from the emitters oftransistors 67 and 69 and is connected to one side of the deflectionyoke coil 81. The other end of the yoke coil 81 is connected to groundthrough sampler resistor 83.

The signal at the ungrounded side of the sampler resistor 83 is fed backto the control signal connected input of differential amplifier 11through series connected resistor 85 and potentiometer 87.

In operation the gross position of the beam on the face of the cathoderay tube is controlled by the coarse position input through resistor 13.After the coarse beam position is determined the symbol input is used tomake the beam trace out the symbol or character.

Both of the input signals are fed into one input of differentialamplifier 11 which provides impedance matching and voltage gain.Differential amplifier 19 further amplifies the output of differentialamplifier 11 and feeds its single ended output into the input of bufferstage 23 which has a high input impedance so as not to load down thedifferential amplifier 19. The output of the buffer stage which is takenfrom the emitter of the emitter follower connected transistor 21 iscoupled to the base of the common emitter connected transistor 29 ofamplifier 31. The output of amplifier 31 can swing almost the full rangebetween :V thereby further providing voltage gain to the output ofbuffer stage 23 which swings between and V. The common emitter connectedamplifier 31 has an inherent inverting function. Therefore, the outputof differential amplifier 19 must be taken from the correct side of theamplifier so that the output of amplifier 31 is of proper polarity todrive in the correct direction the output transistors 67 and 69 throughplus and minus drivers 61 and 51, respectively.

The voltage at the ungrounded ends of capacitors 33 and a very close to+V and V, respectively, since resistors 35 and 37 are quite small andonly a small amount of voltage drops across them. Capacitors 33 and 25perform a regulating function, protecting the circuit from changes invalue of the supply voltages, +V and V. The voltage drops acrossresistors 35 and 37 are so small it will be assumed that the voltages atthe ungrounded ends of capacitors 23 and 35 are equal to theirrespective supply voltages.

When the collector of transistor 29 is positive, output transistor 67 isbiased on through plus driver circuit transistor 59 and outputtransistor 69 if cut-01f. When the voltage at the collector oftransistor 29 is negative, the opposite situation pertains andtransistor 69 is biased on through minus driver circuit transistor 49and transistor 67 is cut-off. If, for instance, the voltage at thecollector of transistor 29 is +5 volts the voltage at the base of theplus driver circuit transistor 59 is at approximately 7.1 volts due tothe voltage drop across the series connected diodes 41, 43 and 45.Therefore, the voltage at the base of transistor 67 is approximately 6.4volts and the voltage at the base of transistor 69 is approximately 5.7volts due to the base-emitter junction voltage drops across transistors59 and 49, respectively. Transistor 67 is biased on and the voltage atits emitter is approximately 5.7 volts. Thus, the voltages at the baseand emitter of transistor 69 are equal and it is nonconducting.

If no input signal is present, the voltage at the collector oftransistor 29 is approximately -1 volt and the voltage at the other endof the series connected diodes is approximately +1 volt. This results ina voltage of approximately +.3 volt at the base of transistor 67 and .3volt at the base of transistor 69 thus causing both output transistorsto be non-conducting and a zero voltage drop to exist across thedeflection yoke 81.

The series connected diodes 41, 43 and 45 are used in the inverter stageto prevent cross-over distortion which would otherwise occur as the yokecurrent goes through zero. The diodes are forward biased when transistor29 is conducting and have a drop of approximately 2.1 volts across them.This compensates for the base to emitter voltage drops across drivertransistors 49 and 59 and output transistors 67 and 69.

The detailed circuitry of differential amplifiers 11 and 19; has notbeen shown since they are well known in the art. Any type can be used aslong as it has a quiescent output voltage level such that under nosignal conditions there is no current through the yoke and the beam isundeflected. They should also have a high input impedance to preventloading on the input circuits and of differential amplifier 19 ondifferential amplifier 19 on differential amplifier 11. This may beobtained by including emitter follower connected transistors in theirinput circuits.

Feedback resistors and 87 are included to control the overall gain ofthe amplifier. The open-loop gain of the circuit is extremely large sothat even a very small input signal would generate a large yoke currentwith the result that the system would be extremely sensitive to noisevoltages and almost impossible to control. The inclusion of the feedbackresistors lowers the gain to such a point that, if the series resistanceof potentiometer 87 and resistor 85 is equal to the input resistors 13or 15, the voltage at the ungrounded end of sampler resistor 83 isapproximately equal to the input voltage. Therefore the amount ofcurrent through deflection yoke 81 is determined by the value of samplerresistor 83. The closed loop voltage gain of the system may becontrolled by adjusting the potentiometer 87.

.Emitter follower connected driver circuits 51 and 61 are used toprevent voltage changes caused by the reactive impedance of yoke 81 frombeing reflected back to the high voltage gain portions of the amplifier.Such reflection back might cause improper operation of the circuit.

With the component values as listed below, the circuit is capable ofsupplying current between +4 amperes and --4 amperes to the deflectionyoke 81.

The following component values have been found to be suitable for use inthe circuit.

Component: Value Resistors- 14 2K Transistor 21 '2N3904 Capacitor 25 /Lf200 Resistor 27 1K Transistor 29 2N698 Capacitor 33 ,u.f 200 Resistors-Transistor 49 2N4036 Capacitor 53 ,u,uf. 390 Resistors 57 6800Transistor 59 2N698 Resistors- 65 1 6800 Transistors 67 2N3715 69 2N379lResistors- 77 2 Deflection yoke 81 h 70 Resistors 83 1 85 3900Potentiometer 81 1K :V v :22 :V v :26

All values of resistance are given in ohms, or kilo-ohms Wherespecified.

With these component values the small signal bandwidth is in excess ofone megacycle for current outputs in the order of 50 milliamperespeak-to-peak.

It should be understood that the above description of the particularconfiguration and environment of this invention is by way ofillustration only and that the circuit clearly could find uses in fieldsother than deflection amplifiers for cathode ray tubes.

What is claimed is: 1. A solid-state amplifier for supplying acontrollable current to an output impedance in response to an inputvoltage signal comprising:

amplification means for providing amplification to said input signal;

output means including two push-pull connected oppositely conductivetransistors having their emitters connected to one end of said outputimpedance for supplying output current to said impedance;

separate transistor driver means for each of said push pull connectedtransistors for supplying input drive current to said push-pullconnected transistors, the input of said separate driver means beingcoupled to the output of said amplification means; and

means included in said amplification means for providing a voltagedifference between the inputs of said separate driver means forcompensating for different voltage drops in said driver means and outputmeans.

2. The amplifier of claim 1 wherein said amplification means includes:

differential amplifier means for amplifying said input signal;

high input impedance buffering means connected to the output of saiddifferential amplifier means for providing isolation and currentamplification to the output signal of said differential amplifier means;and

common emitter connected transistor amplifier means coupled to saidbuffering means and having said means for providing a voltage differencebetween the inputs of said separate driver means connected in itscollector circuit.

3. The amplifier of claim 1 wherein said means for providing a voltagedifference between the inputs of said separate driver means comprisesmeans for maintaining a substantially constant voltage differencebetween said driver inputs.

4. The amplifier of claim 3 wherein said means for maintaining asubstantially constant voltage difference between the inputs of theseparate driver means includes a plurality of series-connectedforward-biased diodes.

5. The amplifier of claim 1 wherein said separate driver means eachinclude emitter-follower connected transistors each of them being of thesame conductivity type as the output means transistor to which it iscoupled.

6. The amplifier of claim 1 further comprising degenerative feedbackmeans coupled between the other end of said output impedance and theinput of said amplification means and including a potentiometerconnected for varying the gain of said amplifier.

7. A. solid-state deflection amplifier for supplying a controllablecurrent to the deflection yoke of a cathode ray tube in response to aninput signal comprising:

differential amplifier means for amplifying said input signal;

amplifying means connected to the output of said differential amplifiermeans for providing amplification to the output signal of saiddifferential amplifier means; an output circuit including two push-pullconnected oppositely conductive transistors having their emittersconnected to one end of said deflection yoke for supplying current ineither direction to said yoke;

separate transistor driver circuits for each of said pushpull connectedtransistors for supplying driving power to said output circuittransistors, the inputs of said driver circuits being connected to saidamplifying means;

means included in said amplifying means for providing a voltagedifference between the inputs of said separate driver circuits forcompensating for different voltage drops in said driver and outputcircuit transistors; and

degenerative feedback means connected between the other end of saiddeflection yoke and'the input of said differential amplifier means forcontrolling the gain and frequency response of the deflection amplifier.

8. The amplifier of claim 7 wherein said means for providing a voltagedifference between the inputs of the separate driver means comprisesmeans for maintaining a substantially constant voltage differencebetween said driver inputs.

9. The amplifier of claim 8 wherein said driver circuits each include anemitter-follower connected transistor of the same conductivity as theoutput circuit transistor to which it is connected and said means formaintaining a substantially constant voltage difference between theinputs of said separate driver' means includes at least oneseries-connected forward-biased diode.

10. A solid state deflection amplifier for supplying a controllablecurrent to the deflection yoke of a cathode ray tube in response toinput voltage signals comprising:

at least one differential amplifier, said input signals being connectedto the input of said differential amplifier;

a common emitter connected transistor amplifier connected to the outputof said amplifier;

an output circuit including two push-pull connected oppositelyconductive transistors having their emitters connected to one end ofsaid deflection yoke for supplying output current to said yoke in eitherdirection;

separate transistor driver circuits connected to the inputs of saidpush-pull connected transistors, the drive circuit transistors being ofthe same conductivity as the output transistors to which they areconnected;

means connected in the collector circuit of said common emitteramplifier providing a substantially constant voltage difference betweenthe inputs of said separate driver circuits for compensating for voltagedrops in said driver and output circuit transistors; and

References Cited UNITED STATES PATENTS 12/1960 Stanley 3l527 X 10/1967Lee et al. 330-122 X RODNEY D. BENNETT, JR., Primary Examiner BRIAN L.RIBANDO, Assistant Examiner US. Cl. X.R.

